Microporous polyolefin membranes are widely used in separators for lithium batteries, etc., electrolytic capacitor separators, steam-permeable, waterproof clothing, various filters, etc. When the microporous polyolefin membranes are used as battery separators, their performance largely affects the performance, productivity and safety of batteries. Particularly lithium ion battery separators are required to have not only excellent mechanical properties and permeability but also shutdown properties and pore-closing function for stopping a battery reaction at the time of abnormal heat generation, thereby preventing the heat generation, ignition and explosion of the battery, which can be caused by the short-circuiting of external circuits, overcharge, etc.; heat shrinkage resistance, a function of keeping a separator shape to avoid a direct reaction between a cathode material and an anode material even when becoming high temperatures; etc.
In general, microporous membranes made only of polyethylene have low meltdown temperatures, while microporous membranes made only of polypropylene have high shutdown temperatures. Thus proposed is a battery separator formed by a microporous membrane made of polyethylene and polypropylene as main components.
Japanese Patent 3235669, for instance, discloses a battery separator having excellent heat shrinkage resistance and shutdown properties, which comprises at least one first layer made of a polymer selected from low-density polyethylene, an ethylene-butene copolymer and an ethylene-hexene copolymer, and at least one second layer made of a polymer selected from high-density polyethylene, ultra-high-molecular-weight polyethylene and polypropylene.
Japanese Patent 3422496 discloses a battery separator having excellent shutdown properties, which comprises at least one first layer made of a polymer selected from ethylene-butene copolymers, ethylene-hexene copolymers, ethylene-methacrylate copolymers and polyethylene, and at least one second layer made of a polymer selected from polyethylene and polypropylene.
Japanese Patent 2883726 discloses a battery separator having excellent shutdown properties and meltdown properties, which is obtained by simultaneously extruding polypropylene having a melting point of 150° C. or higher and polyethylene having a melting point of 100 to 140° C., monoaxially stretching the resultant laminate film at a temperature in a range from −20° C. to the melting point (Tm0) of polyethylene −30° C., and further stretching it in the same direction at a temperature in a range from Tm0 −30° C. to Tm0 −2° C. to make it porous.
JP 11-329390 A proposes a battery separator having excellent shutdown properties and strength, which is formed by a microporous membrane comprising two high-strength, microporous polypropylene layers, and a filler-containing, shutting polyethylene layer, which is sandwiched by the polypropylene layers, the filler-containing, shutting polyethylene layer being produced by a method of stretching a particles-containing film.
As a microporous polyolefin membrane having excellent safety and strength, JP 2002-321323 A proposes a microporous polyolefin membrane obtained by integrally laminating a microporous membrane A comprising polyethylene and polypropylene as indispensable components, and a microporous polyethylene membrane B, in a three-layer structure of A/B/A or B/A/B.
However, polypropylene-containing microporous membranes have poor permeability and pin puncture strength. In addition, recently gaining importance as separator characteristics are not only permeability and mechanical strength, but also battery life characteristics such as cycle characteristics and battery productivity such as electrolytic solution absorbability. Particularly a lithium ion battery electrode expands by the intrusion of lithium when charged, and shrinks by the departure of lithium when discharged, an expansion ratio when charged tending to become larger as recent increase in the capacity of batteries. Because a separator is compressed when the electrode expands, the separator is required to suffer only small permeability variation by compression and have deformability to absorb the expansion of an electrode. However, each microporous membrane described in the above references does not have sufficient compression resistance. A microporous membrane with poor compression resistance is highly likely to provide batteries with insufficient capacity (poor cycle characteristics) when used as a separator.
Thus, the applicant proposed a microporous membrane comprising a polyolefin and a thermoplastic resin other than a polyolefin (for instance, polybutylene terephthalate), fine particles of 1 to 10 μm in diameter based on the thermoplastic resin other than the polyolefin being dispersed in the polyolefin, in which fibrils are cleft with the fine particles as nuclei, thereby forming creased gaps constituting pores containing the fine particles (JP 2004-149637 A). The applicant also proposed a microporous membrane comprising (a) polyethylene, and (b) a thermoplastic resin other than polyethylene (for instance, polymethylpentene-1) having a melting point or a glass transition temperature of 170 to 300° C., which is not completely dissolved but finely dispersed when melt-blended with polyethylene and its solvent, the air permeability increase of the microporous membrane by heat compression at a pressure of 5 MPa and 90° C. for 5 minutes being 500 seconds/100 cm3 or more (JP 2004-161899 A). However, each of the microporous membranes described in these references does not have satisfactory mechanical properties and shutdown speed, with small deformation when compressed.
Accordingly desired is a microporous polyethylene membrane for battery separators, which has a layer comprising a polyethylene resin and a heat-resistant polymer other than polypropylene, so that it has well-balanced shutdown properties, meltdown properties, permeability, mechanical strength, heat shrinkage resistance and compression resistance.